The CDC now says 436 people have been diagnosed with a laboratory-confirmed intestinal illness linked to salads sold at McDonald’s.

The cyclospora infection is part of an ongoing outbreak that began in May.

Last week, the US Food and Drug Administration issued a statement confirming 395 illnesses. That number increased to 436 this week.

At least 20 people have been hospitalized; no deaths have been reported.

The cyclospora parasite causes intestinal illness in people who consumed contaminated food or water. Symptoms can begin a week or more after consuming the parasite. They include diarrhea and frequent, sometimes explosive bowel movements, according to the Centers for Disease Control and Prevention. Those who are infected might also experience loss of appetite, weight loss, stomach cramps or pain, nausea, gas, and fatigue. Vomiting, headache, fever, body aches and flu-like symptoms can also occur.

The illness can last a few days to a few months, and patients might feel better but then get worse again. Cyclosporiasis can be treated with antibiotics.

Illnesses have been reported in 15 states: Connecticut, Florida, Iowa, Illinois, Indiana, Kentucky, Michigan, Minnesota, Montana, Nebraska, Ohio, South Dakota, Tennessee, Virginia, and Wisconsin. However, the patients in Connecticut, Michigan, Tennessee, and Virginia bought their salads in Illinois while traveling, and the Florida patient bought the salad in Kentucky.

The FDA said an unused package of a Fresh Express salad mix of romaine lettuce and carrots that had been distributed to McDonald’s tested positive for the presence of Cyclospora though it was past its July 19 expiration date.

“Epidemiologic evidence indicates that salads purchased from McDonald’s restaurants are one likely source of these infections. The investigation is ongoing, and FDA is working to determine the sources of the ingredients that were in common to the salads served at McDonald’s,” the CDC said in an outbreak update posted on its website Thursday.

McDonald’s said in a statement on July 13 that it had removed the salad mix from it’s impacted restaurants and replaced it with a different supplier.

In a recall statement on the Fresh Express website, the company said it had launched a “precautionary recall” of the institutional food service product on July 27 of a “limited number of cases.”

The company’s salad mix recall was connected with a public health alert issued on July 30 by the US Department of Agriculture’s Food Safety and Inspection Service advising consumers that certain salad and wrap products might be contaminated with cyclospora. The products were sold by grocery stores including Kroger, Trader Joe’s and Walgreens.

In the alert, the USDA said the issue was discovered when Indianapolis-based food distributor Caito Foods “received notification from their lettuce supplier, Fresh Express, that the chopped romaine that is used to manufacture some of their salads and wraps was being recalled.”

The lettuce from McDonald’s that tested positive for the cyclospora parasite was in the same lot as the lettuce distributed to Caito Foods, the CDC said Wednesday. “Fresh Express also reported that no romaine lettuce from the lot that was positive for cyclospora was packaged for direct retail sale to consumers,” the CDC said.